The present invention is directed to methods for increasing the intensity, degree or range of feeding of herbivorous fish. More particularly, the instant invention is directed to methods for stimulating or enhancing feeding in such fish and to compositions useful for causing such stimulation or enhancement.
It has been generally recognized that chemical control of feeding can be an important modulator of physiological well-being in many animal species. Substances present in the diet of an animal may act as feeding stimulants to attract the attention of the animal to a foodstuff or to encourage foraging. Substances may also act as a feeding enhancer to induce an animal to eat more than it normally would or to eat foodstuffs which are not normally consumed or preferred by that animal. The presence or absence of these substances in the diet may determine whether a food item is eaten or rejected by an animal and, to some extent, the quantity consumed. In spite of this general knowledge, however, experimental knowledge which would permit ellucidation of chemical regulation mechanisms of herbivorous fish feeding is sparse.
Fish are believed to be able to detect food by a variety of sensory mechanisms including vision and the chemical senses of olfaction and taste. Taste is believed generally to play a key role in determining whether a potential food item is swallowed or rejected. While chemoreception as a general phenomenon and chemical mediation of gustation and olfaction in particular have been investigated for several species of carnivorous fish, the taste properties and preferences of herbivorous fish have been almost totally ignored.
A. M. Mackie in "Identification of the Gustatory Feeding Stimulants", Chemoreception in Fishes, T. J. Hara, ed., Elsevier New York (1982), pp. 275-291 has concluded that the taste system of carnivorous fish appears to be well-tuned for detecting the presence of certain animal tissue via amino acid content. It was concluded that due to the chemical similarity of prey items in carnivores' diet, their taste receptors need not have the ability to respond to a variety of chemicals having widely differing structures; most of the information concerning the suitability of a food item is believed able to be conveyed by a few amino acids.
The taste system of herbivorous fish, however, is expected to be more complex since the diet of such fish is restricted to plant matter. Accordingly, while amino acids of plant tissue might be expected to be sufficient stimulus to attract herbivorous fish, it is expected that the fish, once attracted, might use a second sensitivity such as, for example, the response to a bitter taste of an alkaloid toxin, to avoid ingesting plants which produce potentially harmful physiological effects. For example, it has been found by Sun et al. in "Rhipocephalin and Rhipocephalinal; Toxic Feeding Deterrents From the Tropical Marine Alga Rhipocephalus phoenix", Tetrahedron Letters, pp. 685-688 (1979) that when certain plant-derived feeding deterrents were incorporated into food pellets, the pellets were rejected by certain species of herbivorous fish.
Despite the apparent complexities of studying the chemosensory behavior of herbivorous fish, a long-felt need exists for feeding stimulants and enhancers for herbivorous fish since herbivorous fish are widely employed in aquaculture as a human food source. It is generally acknowledged that animals low on the food chain, i.e. herbivores, can be produced much more economically than can animals higher on the food chain i.e., carnivores. By having an understanding of the regulatory mechanism of food intake in herbivorous fish, and by having knowledge of methods for increasing feeding in such fish and for compositions capable of stimulating such increase, it would likely be possible to manipulate the feeding behavior of the fish in a manner that would increase food consumption and growth, would encourage feeding on low quality foodstuffs, and would, accordingly, increase the food supply available to humanity.